1. Technical Field of the Invention
The present invention relates generally to spark plugs for use in internal combustion engines of automobiles and cogeneration systems.
More particularly, the invention relates to a spark plug with multiple ground electrodes, which has a high capability to ignite the air-fuel mixture (to be referred to as ignition capability hereinafter) and a long service life.
2. Description of the Related Art
FIG. 36 shows an existing spark plug 9 for use in an internal combustion engine of an automobile, which includes a tubular metal shell 94, an insulator 92, a cylindrical center electrode 93, a first ground electrode 951, and a pair of second ground electrodes 952.
(For example, Japanese Patent No. 3140006, an English equivalent of which is U.S. Pat. No. 6,229,253, discloses such a multiple ground electrodes spark plug.)
The tubular metal shell 94 has a threaded portion 940 for fitting the spark plug 9 into the combustion chamber of the engine.
The insulator 92 has a bore 921 formed therethrough and is fixed in the metal shell 94 such that an end 922 thereof protrudes from an end 941 of the metal shell 94.
The cylindrical center electrode 93 is secured in the bore 921 of the insulator 92 and has an end 931 that protrudes from the end 922 of the insulator 92.
The first ground electrode 951 has a base end joined to the end 941 of the metal shell 94 and a tip portion that faces the end 931 of the center electrode 93 through a first spark gap A in the axial direction of the center electrode 93.
Each of the second ground electrodes 952 has a base end joined to the end 941 of the metal shell 94 and a tip portion that faces the outer side surface of the center electrode 93 through a second spark gap G in the radial direction of the center electrode 93.
In such a spark plug 9, normal sparks are discharged across the first spark gap A in normal condition of the spark plug.
However, when the combustion temperature of the air-fuel mixture is low, the temperature on the outer surface of the insulator 92 will be accordingly low. As a result, carbon will deposit on the outer surface of the insulator 92, thus causing “carbon-fouling” of the insulator 92.
Generally, the carbon-fouling of an insulator in a spark plug causes the insulation resistance between a center electrode and a metal shell of the spark plug to be decreased, and even results in misfire of the engine.
However, in the spark plug 9, when the insulator 92 is fouled with carbon, instead of normal sparks, “side sparks” are discharged across the second spark gaps G, thereby burning off the carbon deposit on the outer surface of the insulator 92.
As a result, the outer surface of the insulator 92 is cleaned by the spark plug 9 itself, thus recovering the insulation resistance and ignition capability of the spark plug 9.
However, at the same time, side sparks may cause a “channeling problem” and wear on the outer side surface of the center electrode 93. The channeling problem here denotes a phenomenon in which the heat energy transferred from side sparks to the end 922 of the insulator 92 partially melts the insulator 92, thereby forming channels on the end 922 of the insulator 92.
Accordingly, it is required for the spark plug 9 to discharge side sparks only when the insulator 92 is fouled with carbon, so that the durability and ignition capability of the spark plug 9 can be secured.
However, when the spark plug 9 is used in a recent engine, which is of high-compression and lean-burn type and used in combination with a supercharger and an EGR (Exhaust Gas Recirculation) system, the flow speed in the proximity of the spark plug 9 is high. (Such an engine will be referred to as a high flow speed engine hereafter.)
The high flow speed will force normal sparks discharged in the first spark gap A of the spark plug 9 to deviate from the normal course (i.e., in the axial direction of the center electrode 93), thus increasing the required voltage of the spark plug 9 for discharging normal sparks.
As a result, even when the insulator 92 is not fouled with carbon, occurrence rate of side sparks will increase, so that formation of channels on the insulator 92 and wear on the outer side surface of the center electrode 93 can be facilitated, thus shortening the service life of the spark plug 9.
To solve the above problem, one may consider taking a countermeasure to decrease the required voltage of the spark plug 9 for discharging normal sparks, thereby reducing the occurrence rate of side sparks. However, at the same time, decreasing the required voltage for discharging normal sparks may cause the occurrence rate of side sparks to be decreased even when the insulator 92 is fouled with carbon, thus decreasing the self-clean capability of the spark plug 9.